Although heterosexual transmission is the most important manner by which HIV is spread throughout the world today, very little is known about the mechanisms involved in the initiation and establishment of infection at the mucosal surfaces of the genital tract. The primary goal of this project is to determine if specific genotypes of SIV are preferentially transmissible by the mucosal route. During the past year we have investigated the influence of cell tropism on mucosal transmission. We have found essentially no difference in efficiency of vaginal transmission between lymphotropic (SIVmac239) and macrophage tropic (SIVmac239/17E and SIVsmmPGm5.3) viruses. The results of this work will lead to a better understanding of the processes involved in sexual transmission of HIV, with the goal of designing vaccines which will prevent infection.